It is conventional practice to heat formations containing heavy oil or tars to stimulate the extraction. The objective is suitably to lower the viscosity of the oils and tars so they can flow through the formation and the well.
Steam injection is widely used for this purpose. Its applicability is not universal, however. There are many constraints which render steam injection either unsuccessful or only marginally successful. One is that the maximum temperature rise attainable with steam injection may not be sufficient to reduce the viscosity of very heavy oils and tars enough to enable them to move at acceptable rates. Another is the tendency of steam (or heated air when used) to override the oil--that is, rise to the top of it. Heat transfer to the oil is much less effective when the steam is on top than if the heat is applied within or below the oil or tar.
Some of the shortcomings of steam injection can be overcome by a different class of process, in which combustion is conducted within the formation itself--in-situ combustion (ISC). Higher temperature combustion processes can be employed with these processes. In some of these processes, the fuel for combustion is injected into the formation and combusted in-sutu. In others, the formation itself is ignited and part of it is combusted. In this latter procedure, part of the oil or tar is sacrificed to generate the heat. Poettmann, U.S. Pat. No. 3,127,935 provides examples of both of these techniques. This invention constitutes an improved in-situ combustion process.
A considerable disadvantage of steam injection, and also of injection of air and air-fuel mixtures into a formation is the cost of necessary fixed installations such as boilers, pipelines, and compressors, and costs of energy to operate them. A further disadvantage of these systems is that they frequently consume, on an energy balance basis, about one barrel of oil for each two barrels of oil produced, and generally derive this energy from fossil fuels.
It is an object of this invention to heat a formation by ISC which does not require air to be injected to maintain the reaction, nor combustion of the resource to be extracted, nor the supply of any gaseous fuel.
It is another object of this invention to provide an ISC process in which gases that are generated by it can improve both the physical and chemical properties of the oils extracted from the well.